Provide more information for failures involving FindFocus()

It is useful to know where the focus actually is when a test checking
that it's set to a particular window fails.

tests/testwindow.h

@@ -0,0 +1,61 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        tests/testwindow.h
+// Purpose:     Unit test helper for comparing wxWindow pointers.
+// Author:      Vadim Zeitlin
+// Copyright:   (c) 2019 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_TESTS_TESTWINDOW_H_
+#define _WX_TESTS_TESTWINDOW_H_
+
+#include "wx/window.h"
+
+// We need to wrap wxWindow* in a class as specializing StringMaker for
+// wxWindow* doesn't seem to work.
+class wxWindowPtr
+{
+public:
+    explicit wxWindowPtr(wxWindow* win) : m_win(win) {}
+
+    wxString Dump() const
+    {
+        if ( !m_win )
+            return "(no window)";
+
+        wxString s = m_win->GetClassInfo()->GetClassName();
+        const wxString& label = m_win->GetLabel();
+        if ( !label.empty() )
+        {
+            s += wxString::Format(" (label=\"%s\")", label);
+        }
+
+        return s;
+    }
+
+private:
+    friend bool operator==(wxWindowPtr wp1, wxWindowPtr wp2)
+    {
+        return wp1.m_win == wp2.m_win;
+    }
+
+    wxWindow* const m_win;
+};
+
+// Macro providing more information about the current focus if comparison
+// fails.
+#define CHECK_FOCUS_IS(w) CHECK(wxWindowPtr(wxWindow::FindFocus()) == wxWindowPtr(w))
+
+namespace Catch
+{
+    template <>
+    struct StringMaker<wxWindowPtr>
+    {
+        static std::string convert(const wxWindowPtr window)
+        {
+            return window.Dump().ToStdString();
+        }
+    };
+}
+
+#endif // _WX_TESTS_TESTWINDOW_H_